Various types of radiation monitoring stations are used to inspect a stream of vehicles. shipping containers, packages, luggage, persons, animals, manufactured products, process materials, waste materials, and similar items to determine whether any radiological material is present therein. In such systems a flow of articles to be inspected is typically configured to pass by or through the monitoring station. For example, pedestrian portal monitors may be designed for detection of microcurie-levels of special nuclear materials. In a conventional designs the portal contains one or two large gamma detectors, usually plastic scintillators or NaI (Tl) detectors on each side of the portal. The detectors are generally designed so that they issue an alarm for a count rate that is statistically above the background count rate which would be detected when no radiation sources are near the portal. The statistically significant (alarm threshold) level may be calculated from cumulative probability distributions. A complicating effect that is common with such monitoring stations is that radiological sources that are in the queue to pass by or through the radiation monitoring station may trigger an alarm in the station before the article or material actually bearing the radiological source reaches the station. As a consequence, material that has reached the station ahead of the actual radiological source may be falsely identified as containing the radiological source.
One example illustrating this problem is a radioactive source present in a medical patient who has received a radionuclide administration and is in a queue approaching a pedestrian radiation monitoring portal. The radioactivity levels of many medical radionuclides are a thousand to more than ten thousand times the radioactivity level that pedestrian portal systems are typically designed to detect. Because the portal has typically been designed to detect much smaller radioactive sources, a nearby medical radioisotope source generally leads to detector count rates that are above the alarm threshold when the large activity medical source is still a long distance (often 5 to 10 meters) from the portal. If a continuous queue of pedestrians (vehicles, etc.) is moving through the portal then the alarm will appear to be due to the person (vehicle, etc.) within the portal, but the alarm is actually caused by a radiation source further back in the queue. All persons/things in the queue will appear to generate an alarm until the large source has cleared the portal or moved sufficiently far from the portal that it no longer causes an increase in detector count rate.
Thus, in this example, the challenge is how to avoid detaining all of the people who are in front of the medical patient and who innocently set off the alarm because they are occupying the portal when the count rate is significantly elevated due to the high gamma rate emissions from the medical patient who is in the queue behind them. The resulting interdiction of innocent persons may require secondary inspections involving specially-trained enforcement personnel and radionuclide identifier equipment. This procedure involves labor and equipment costs that are both undesirable and, for the circumstances described in this example, are unnecessary. Clearly this is not a satisfactory operating mode.
Various techniques have been developed in attempts to overcome the effects of premature detection of radiological materials in radiation monitoring stations. For example, the sensitivity of radiation detectors in the radiation monitoring station may be decreased to minimize premature detection. However this has the disadvantage of potentially failing to detect a small quantity of offensive radioactive material as it passes through the radiation monitoring station. Another technique that has been attempted is to shield the radiation detectors so that they only “see” radiation sources that are located within a specific defined small viewing angle. However, because such shields limit the viewing window to a relatively small angle, the shields must screen off very large angles around the detectors. Furthermore, because the detectors are very sensitive, being typically designed to detect very small quantities (e.g., micro-curies) of special nuclear material, the shields must be very bulky in order to prevent the undesired detection of larger quantities of special nuclear material that are outside the specified small viewing angle. This shielding bulk adds considerable undesired weight and cost to the monitoring station.
What are needed therefore are methods and systems for rejecting radioactive interference in radiation monitoring stations from nearby radiation sources.